Disinfection, as applied in water and wastewater treatment, is a process by which pathogenic microorganisms are inactivated to provide public health protection. Chlorination has been the dominant method employed for disinfection for almost 100 years. However, it is no longer the disinfection method automatically chosen for either water or wastewater treatment because of the potential problems associated with disinfection by-products and associated toxicity in treated water. Ultraviolet (UV) irradiation is a frequent alternative chosen to conventional chlorination. Since UV radiation is a nonchemical agent, it does not yield disinfectant residual. Therefore, concerns associated with toxic disinfectant residuals do not apply. In addition, UV disinfection is a rapid process. Little contact time (on the order of seconds rather than minutes) is required. The result is that UV equipment occupies little space when compared to chlorination and ozonation.
The responses of microorganisms to UV irradiation are attributable to the dose of radiation to which they are exposed. The UV dose is defined as the product of radiation intensity and exposure time. As a result of turbulent flow conditions and three-dimensional spatial variations in UV intensity, continuous-flow UV systems deliver a broad distribution of UV does. Principles of reactor theory can be used to demonstrate that this distribution of doses leads to inefficient use of the UV energy emitted within these systems. Furthermore, the theoretical upper limit on UV reactor performance coincides with a system which accomplishes the delivery of a single UV dose (i.e., a dose distribution which can be represented by a delta function). Optimal dose distribution is difficult to achieve in currently used UV disinfection systems.
An average dose does not accurately describe the disinfection efficiency of a full-scale UV system. UV intensity is a function of position. The intensity of UV radiation decreases rapidly with distance from the source of radiation. Exposure time is not a constant either. The complex geometry of UV systems dictates complex hydrodynamic behavior as well, with strong velocity gradients being observed. Coincidentally, fluid velocity is generally highest in areas of lowest intensity. This creates a situation in which some microorganisms are exposed to a low UV intensity over a comparatively short period of time, thereby allowing them to “escape” the system with a relatively low UV dose. This represents a potentially serious process limitation in UV systems. For example, if 1% of the microorganisms received doses lower than the lethal level, then the maximum inactivation achievable by the system is 99%, no matter what actual average dose was delivered.
Non-uniform distribution of UV doses in systems indicates that UV radiation is applied inefficiently. While UV overdose apparently presents no danger in terms of finished water composition, it does increase operating and capital costs. Therefore, it is desirable to have a system which incorporates the effects of hydrodynamic behavior and the UV intensity field to provide for complete disinfection.
Mathematical modeling of UV reactors has been used to improve reactor design and predict microbial inactivation. Do-Quang et al (1997) discussed the use of CFD modeling of a vertical lamp open channel UV reactor to improve microbial inactivation. Blatchley et al (1997) proposed a method of particle tracking for calculating the dose distribution in an open channel horizontal lamp UV reactor. That model took into consideration both the hydrodynamics of the system and the intensity field.
The above models were based on low-pressure lamp systems installed in wastewater that have a single germicidal wavelength output at 254 nm. CFD modeling is helpful in assessing reactor design with in-line drinking water reactors since the hydraulic residence time in these systems is less than about 1 second and the spacing between lamps is larger and non-uniform in comparison to most low-pressure lamp wastewater systems.